The present invention relates to a photopolymerizable mixture which contains a polymeric binder, a polymerizable compound having at least two acrylate or alkacrylate groups in the molecule, and a photo-initiator combination.
Photopolymerizable mixtures which, in order to increase the photosensitivity, contain certain combinations of photoinitiators and activators, for example carbonyl group-containing initiators and tertiary amines, are known. Such mixtures with a synergistic effect are described, for example, in DE-A 2,602,419 and 2,251,048 and in US-A 3,759,807. A disadvantage of these mixtures, which contain low-molecular-weight amines, is that they have a short shelf life since the amines can easily bleed out, in particular from thin layers.
In JP-A 50/129,214, registered on 2nd April 1974 under the number 49/36,614, a photopolymerizable mixture is described which contains a tetra(meth)acrylate of a N,N,N',N'-tetrahydroxyalkylalkylenediamine as the polymerizable compound. The tetrafunctional compound functions as a crosslinking agent.
Mixtures are also known which contain triethanolamine trimethacrylate as crosslinking agent; they are described for example, in Chem. Abstr. 86 (1977), 30443q and 95 (1981), 220 727u. These compounds also tend to diffuse and evaporate out of the coating on relatively long storage and at elevated ambient temperatures.
It is furthermore known that the free-radical polymerization of ethylenically unsaturated compounds can be initiated by irradiation with visible light in the presence of photoreducible dyes and reducing agents, for example amines (US-A 3,097,096). However, these initiator combinations are essentially employed only in aqueous solution or in combination with water-soluble binders.
Initiator combinations of photoreducible dyes and other reducing agents are described in US-A 3,597,343 and 3,488,269. Photopolymerizable mixtures which exclusively contain photoreducible dyes as initiators have hitherto not been employed in practice due to their inadequate photosensitivity.
In JP-A 54/151,024, a photopolymerizable mixture is described which contains an initiator combination of a merocyanine dye and a trihalomethyl-substituted s-triazine and is sensitive towards visible light, for example an argon laser. However, the sensitivity of these mixtures towards visible laser light is inadequate for economic utilization.